Optical sampling interface system for in vivo measurement of tissue

ABSTRACT

An optical sampling interface system minimizes and compensates error resulting from sampling variations and measurement site state fluctuations. Components include: An optical probe placement guide having an aperture wherein the optical probe is received, facilitates repeatable placement accuracy on surface of a tissue measurement site with minimal, repeatable disturbance to surface tissue. The aperture creates a tissue meniscus that minimizes interference due to surface irregularities and controls variation in tissue volume sampled; an occlusive element placed over the tissue meniscus isolates the meniscus from environmental fluctuations, stabilizing hydration at the site and thus, surface tension; an optical coupling medium eliminates air gaps between skin surface and optical probe; a bias correction element applies a bias correction to spectral measurements, and associated analyte measurements. When the guide is replaced, a new bias correction is determined for measurements done with the new placement. Separate components of system can be individually deployed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of U.S. patent application Ser. No.10/170,921 filed Jun. 12, 2002, which is a Continuation-in-Part of U.S.patent application Ser. No. 09/563,782 filed May 2, 2000, now U.S. Pat.No. 6,415,782 (Jul. 2, 2002), which is hereby incorporated by referencein its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to optical sampling of tissue in vivo. Moreparticularly, the invention relates to an optical sampling interfacesystem that includes an optical probe placement guide, a means forstabilizing the sampled tissue, an optical coupler for repeatablysampling a tissue measurement site in vivo, and a means for compensatingmeasurement bias.

2. Technical Background

In vivo measurement of tissue properties and analytes using opticallybased analyzers requires that tissue measurement region be positionedand coupled with respect to an optical interface or probe. Therequirements of an optical sampling interface system for such placementand coupling would depend upon the nature of the tissue properties andanalytes under consideration, the optical technology being applied andthe variability of the tissue with respect to the target analyte. Often,when sampling reproducibility is vital, the optical measurement isperformed in a laboratory where the majority of the factors pertainingto the measurement can be controlled or constrained. However, there aremany demanding in vivo applications that cannot be performed in alaboratory setting but yet require a high degree of optical samplereproducibility. Often, a relatively unskilled operator or user mustperform the optical measurement. One such application is thenon-invasive measurement of glucose through near-infrared spectroscopy.With the desired end result being an optical measurement system that canbe used by the consumer in a variety of environments, the opticalsampling requirements are stringent. This problem is further consideredthrough a discussion of the target application, the structure of liveskin and the dynamic properties of live tissue.

Noninvasive Measurement of Glucose

Diabetes is a leading cause of death and disability worldwide andafflicts an estimated 16 million Americans. Complications of diabetesinclude heart and kidney disease, blindness, nerve damage, and highblood pressure with the estimated total cost to United States economyalone exceeding $90 billion per year. See Diabetes Statistics,Publication No. 98-3926, National Institutes of Health, Bethesda Md.(November 1997). Long-term dinical studies show that the onset ofcomplications can be significantly reduced through proper control ofblood glucose levels. See The Diabetes Control and Complications TrialResearch Group, The effect of intensive treatment of diabetes on thedevelopment and progression of long-term complications ininsulin-dependent diabetes mellitus, N Eng J of Med, 329:977-86 (1993).A vital element of diabetes management is the self-monitoring of bloodglucose levels by diabetics in the home environment. A significantdisadvantage of current monitoring techniques is that they discourageregular use due to the inconvenience and pain involved in drawing bloodthrough the skin prior to analysis. Therefore, new methods forself-monitoring of blood glucose levels are required to improve theprospects for more rigorous control of blood glucose in diabeticpatients.

Numerous approaches have been explored for measuring blood glucoselevels, ranging from invasive methods such as microdialysis tononinvasive technologies that rely on spectroscopy. Each method hasassociated advantages and disadvantages, but only a few have receivedapproval from certifying agencies. To date, no noninvasive techniquesfor the self-monitoring of blood glucose have been certified.

One method, near-infrared spectroscopy involves the illumination of aspot on the body with near-infrared electromagnetic radiation (light inthe wavelength range 700-2500 nm). The light is partially absorbed andscattered, according to its interaction with the tissue constituentsprior to being reflected back to a detector. The detected light containsquantitative information that is based on the known interaction of theincident light with components of the body tissue including water, fat,protein, and glucose.

Previously reported methods for the noninvasive measurement of glucosethrough near-infrared spectroscopy rely on the detection of themagnitude of light attenuation caused by the absorption signature ofblood glucose as represented in the targeted tissue volume. The targetedtissue volume is that portion of irradiated tissue from which light isreflected or transmitted to the spectrometer detection system. Thesignal due to the absorption of glucose is extracted from the spectralmeasurement through various methods of signal processing and one or moremathematical models. The models are developed through the process ofcalibration on the basis of an exemplary set of spectral measurementsand associated reference blood glucose values (the calibration set)based on an analysis of capillary (fingertip) or venous blood.

Near-infrared spectroscopy has been demonstrated in specific studies torepresent a possible approach for the noninvasive measurement of bloodglucose levels. M. Robinson, R. Eaton, D. Haaland, G. Keep, E. Thomas,B. Stalled, P. Robinson, Noninvasive glucose monitoring in diabeticpatients: A preliminary evaluation, Clin Chem, 38:1618-22 (1992) reportsthree different instrument configurations for measuring diffusetransmittance through the finger in the 600-1300 nm range. Mealtolerance tests were used to perturb the glucose levels of threesubjects and calibration models were constructed specific to eachsubject on single days and tested through cross-validation. Absoluteaverage prediction errors ranged from 19.8 to 37.8 mg/dL. H. Heise, R.Marbach, T. Koschinsky, F. Gries, Noninvasive blood glucose sensorsbased on near-infrared spectroscopy, Artif Org, 18:439-47 (1994); H.Heise, R. Marbach, Effect of data pretreatment on the noninvasive bloodglucose measurement by diffuse reflectance near-IR spectroscopy, SPIEProc, 2089:114-5 (1994); R. Marbach, T. Koschinsky, F. Gries, H. Heise,Noninvasive glucose assay by near-infrared diffuse reflectancespectroscopy of the human inner lip, Appl Spectrosc, 47:875-81 (1993)and R. Marbach, H. Heise, Optical diffuse reflectance accessory formeasurements of skin tissue by near-infrared spectroscopy, AppliedOptics 34(4):610-21 (1995) present results through a diffuse reflectancemeasurement of the oral mucosa in the 1111-1835 nm range with anoptimized diffuse reflectance accessory. In vivo experiments wereconducted on single diabetics using glucose tolerance tests and on apopulation of 133 different subjects. The best standard error ofprediction reported was 43 mg/dL and was obtained from a two-day singleperson oral glucose tolerance test that was evaluated throughcross-validation.

K. Jagemann, C. Fischbacker, K. Danzer, U. Muller, B. Mertes,Application of near-infrared spectroscopy for noninvasive determinationof blood/tissue glucose using neural network, Z Phys Chem, 191 S:179-190 (1995); C. Fischbacker, K. Jagemann, K. Danzer, U. Muller, L.Papenkrodt, J. Schuler, Enhancing calibration models for noninvasivenear-infrared spectroscopic blood glucose determinations, Fresenius JAnal Chem 359:78-82 (1997); K. Danzer, C. Fischbacker, K. Jagemann, K.Reichelt, Near-infrared diffuse reflection spectroscopy for noninvasiveblood-glucose monitoring, LEOS Newsletter 12(2):9-11 (1998); and U.Muller, B. Mertes, C. Fischbacker, K. Jagemann, K. Danzer, Noninvasiveblood glucose monitoring by means of new infrared spectroscopic methodsfor improving the reliability of the calibration models, Int J ArtifOrgans, 20:285-290 (1997) recorded spectra in diffuse reflectance overthe 800-1350 nm range on the middle finger of the right hand with afiber-optic probe. Each experiment involved a diabetic subject and wasconducted over a single day with perturbation of blood glucose levelsthrough carbohydrate loading. Results, using both partial least squaresregression and radial basis function neural networks were evaluated onsingle subjects over single days through cross-validation. Danzer, etal., supra, report an average root mean square measurement error of 36mg/dL through cross-validation over 31 glucose profiles.

J. Burmeister, M. Arnold, G. Small, Human noninvasive measurement ofglucose using near infrared spectroscopy [abstract], Pittcon, NewOrleans La. (1998) collected absorbance spectra through a transmissionmeasurement of the tongue in the 1429-2000 nm range. A study of fivediabetic subjects was conducted over a 39-day period with five samplestaken per day. Every fifth sample was used for an independent test setand the standard error of prediction for all subjects was greater than54 mg/dL.

In T. Blank, T. Rucht, S. Malin, S. Monfre, The use of near-infrareddiffuse reflectance for the noninvasive prediction of blood glucose,IEEE Lasers and Electro-Optics Society Newsletter, 13:5 (October 1999),the reported studies demonstrate noninvasive measurement of bloodglucose during modified oral glucose tolerance tests over a short timeperiod. The calibration was customized for the individual and testedover a relatively short time period.

In all of these studies, diverse limitations were cited that wouldaffect the acceptance of such a method as a commercial product.Fundamental to all the studies is the problem of the small signalattributable to glucose, particularly in view of the difficulty inobtaining a reproducible sample of a given tissue volume, as a result ofthe complex and dynamic nature of the tissue. For example, see O.Khalil, Spectroscopic and dinical aspects of noninvasive glucosemeasurements, Clin Chem, v.45, pp.165-77 (1999). The sampling problem isfurther accentuated by noting that the reported studies were performedunder highly controlled conditions using skilled techniques rather thanin a home environment by the consumer. As reported by S. Malin, T.Ruchti, An Intelligent System for Noninvasive Blood Analyte Prediction,U.S. Pat. No. 6,280,381 (Aug. 28, 2001), the entirety of which is herebyincorporated by reference, chemical, structural and physiologicalvariations occur that produce dramatic and nonlinear changes in theoptical properties of the tissue sample. See R. Anderson, J. Parrish,The optics of human skin, Journal of Investigative Dermatology, 7:1,pp.13-19 (1981); W. Cheong, S. Prahl, A. Welch, A review of the opticalproperties of biological tissues, IEEE Journal of Quantum Electronics,26:12, pp.2166-2185, (December 1990); D. Benaron, D. Ho, Imaging (NIRI)and quantitation (NIRS) in tissue using time-resolved spectrophotometry:the impact of statically and dynamically variable optical path lengths,SPIE, 1888, pp.10-21 (1993); J. Conway, K. Norris, C. Bodwell, A newapproach for the estimation of body composition: infrared interactance,The American Journal of Clinical Nutrition, 40, pp.1123-1140 (December1984); S. Homma, T. Fukunaga, A. Kagaya, Influence of adipose tissuethickness in near infrared spectroscopic signals in the measurement ofhuman muscle, Journal of Biomedical Optics, 1:4, pp.418-424 (October1996); A. Profio, Light transport in tissue, Applied Optics, 28:12), pp.2216-2222, (June 1989), M. Van Gemert, S. Jacques, H. Sterenborg, W.Star, Skin optics, IEEE Transactions on Biomedical Engineering, 36:12,pp.1146-1154 (December 1989); and B. Wilson, S. Jacques, Opticalreflectance and transmittance of tissues: principles and applications,IEEE Journal of Quantum Electronics, 26:12, pp. 2186-2199.

The measurement is further complicated by the heterogeneity of thesample, the multi-layered structure of the skin and the rapid variationrelated to hydration levels, changes in the volume fraction of blood inthe tissue, hormonal stimulation, temperature fluctuations and bloodanalyte levels. This can be further considered through a discussion ofthe scattering properties of skin and the dynamic nature of the tissue.

Structure of Human Skin

The structure and pigmentation of human skin vary widely amongindividuals as well as between different sites on the same individual.Skin consists of a stratified, cellular epidermis, and an underlyingdermis of connective tissue. Below the dermis is the subcutaneous fattylayer or adipose tissue. The epidermis is the thin outer layer thatprovides a barrier to infection and loss of moisture, while the dermisis the thick inner layer that provides mechanical strength andelasticity. The epidermis layer is 10-150 μm thick and can be dividedinto three layers, the basal, middle, and superficial layers. The basallayer borders the dermis and contains pigment-forming melanocyte cells,keratinocyte cells, Langherhan cells and Merkel cells See F. Ebling, Thenormal skin, In: Textbook of Dermatology, A. Rook, D. Wilkinson, F.Ebling, eds., 3ed., pp.5-30, Blackwell Scientific Publishers, Oxford,England (1979). The superficial layer is also known as the stratumcorneum (SC).

The stratum corneum, the outermost layer of the mammalian epidermis, isformed and continuously replenished by the slow upward migration ofaqueous keratinocyte cells from the germinative basal layer of theepidermis. It is replenished about every 2 weeks in mature adults. SeeW. Montagna, The Structure and Function of Skin, 2ed., p.454, AcademicPress, New York, (1961). This complex process involving intracellulardehydration and synthesis of an insoluble protein, keratin, results inkeratin-filled, biologically inactive, shrunken cells. These flat,dehydrated, hexagonal cells are tightly bound to their neighbors andeach is approximately 30 μm wide and 0.8 μm deep. See H. Baker, The skinas a barrier, In: Textbook of Dermatology, A. Rook, D. Wilkinson, F.Ebling, eds., 3ed., pp.5-30, Blackwell Scientific Publishers, Oxford,England (1979). There are about twelve to twenty cell layers over mostof the body surface. The stratum corneum is typically 10-20 μm thick,except on the planar surfaces, where it is considerably thicker. See A.Kligman, The Biology of the stratum corneum, in: The Epidermis, W.Montagna, W. Lobitz, eds. Academic Press, New York, pp. 387-433 (1964).

The major constituent of the dermis, apart from water, is a fibrousprotein, collagen, which is embedded in a ground substance composedmainly of protein and glycosaminoglycans. The glycosaminoglycans play akey role in regulating the assembly of collagen fibrils and tissuepermeability to water and other molecules. See K. Trier, S. Olsen, T.Ammitzboll, Acta. Ophthalmol., v. 69, pp.304-306 (1990). Collagen is themost abundant protein in the human body. Elastin fibers are alsoplentiful though they constitute a smaller proportion of the bulk. Thedermis also contains other cellular constituents and has a very richblood supply, though no vessels pass the dermo-epidermal junction. SeeEbling, supra. The blood vessels nourish the skin and control bodytemperature. In humans, the thickness of the dermis ranges from 0.5 mmover the eyelid to 4 mm on the back and averages approximately 1.2 mmover most of the body. See S. Wilson, V. Spence, Phys. Med. Biol. v.33,pp.894-897 (1988).

FIG. 1 shows a plot of the spectral characteristics of excised skin,with no associated fat 101, pure collagen 102, and beef fat 103. Theprocessed second derivative is used to compare the contributions of fatand collagen with the excised skin, mainly consisting of collagen andwater.

Interaction Between Light and Human Skin

When a beam of light beam is directed onto the skin surface, a part ofit is reflected while the remaining part penetrates the skin. Theproportion of reflected light energy is strongly dependent on the angleof incidence. At nearly perpendicular incidence, about 4% of theincident beam is reflected due to the change in refractive index betweenair (η_(D)=1.0) and dry stratum corneum (η_(D)=1.55). For normallyincident radiation, this “specular reflectance” component may be as highas 7%, because the very rigid and irregular surface of the stratumcorneum produces off-normal angles of incidence. Regardless of skincolor, specular reflectance of a nearly perpendicular beam from normalskin is always between 4-7% over the entire spectrum from 250-3000 nm.See R. Scheuplein, J. Soc. Cosmet. Chem., v.15, pp. 111-122 (1964). Onlythe air-stratum corneum border gives rise to a regular reflection.Results from a previous study indicate that the indices of refraction ofmost soft tissue (skin, liver, heart, etc) lie within the 1.38-1.41range with the exception of adipose tissue, which has a refractive indexof approximately 1.46. See J. Parrish, R. Anderson, F. Urbach, D. Pitts,UV-A: Biologic Effects of Ultraviolet Radiation with Emphasis on HumanResponses to Longwave Ultraviolet, New York, Plenum Press (1978).Therefore, these differences in refractive index between the differentlayers of the skin are too small to give a noticeable reflection. SeeEbling, supra. The differences are expected to be even moreinsignificant when the stratum corneum is hydrated, owing to refractiveindex matching.

The 93-96% of the incident beam that enters the skin is attenuated dueto absorption or scattering within any of the layers of the skin. Thesetwo processes taken together essentially determine the penetration oflight into skin, as well as remittance of scattered light from the skin.Diffuse reflectance or remittance is defined as that fraction ofincident optical radiation that is returned from a turbid sample.Absorption by the various skin constituents mentioned above account forthe spectral extinction of the beam within each layer. Scattering is theonly process by which the beam may be returned to contribute to thediffuse reflectance of the skin. Scattering results from differences ina medium's refractive index, corresponding to differences in thephysical characteristics of the particles that make up the medium. Thespatial distribution and intensity of scattered light depends upon thesize and shape of the particles relative to the wavelength, and upon thedifference in refractive index between the medium and the constituentparticles.

The scattering coefficient of biological tissue depends on manyuncontrollable factors, which include the concentration of interstitialwater, the density of structural fibers, and the shapes and sizes ofcellular structures. Scattering by collagen fibers is of majorimportance in determining the penetration of optical radiation withinthe dermis. See F. Bolin, L. Preuss, R. Taylor, R. Ference, Appl. Opt,v. 28, pp. 2297-2303 (1989). The greater the diffusing power of amedium, the greater will be the absorption related to multiple internalreflections. Therefore, reflectance values measured on different siteson the same person, or from the same site on different people, candiffer substantially even when the target absorber is present in thesame concentration. These differences can be attributed to gender, age,genetics, disease, and exogenous factors due to lifestyle differences.For example, it is known that skin thickness in humans is greater inmales than females, whereas the subcutaneous fat thickness is greater infemales. The same group reports that collagen density, the packing offibrils in the dermis, is higher in the forearms of males than females.See S Schuster, M. Black, E. McVitie, Br. J. Dermatol, v.93, pp.639-643,(1975).

Dynamic Properties of the Skin

While knowledge of and utilization of the properties of the skin, highinstrument sensitivity, and compensation for inherent nonlinearities areall vital for the application of non-invasive technologies tononinvasive tissue analyte measurement, an understanding of biologicaland chemical mechanisms that lead to time dependent changes in theproperties of skin tissue is equally important and, yet, largelyignored. At a given measurement site, skin tissue is often assumed to bestatic except for changes in the target analyte and other interferingspecies. However, variations in the physiological state and fluiddistribution of tissue profoundly affect the optical properties oftissue layers and compartments over a relatively short period of time.Such variations are often dominated by fluid compartment equalizationthrough water shifts and are related to hydration levels and changes inblood analyte levels.

Total body water accounts for over 60% of the weight of the averageperson and is distributed between two major compartments: theintracellular fluid (two-thirds of total body water) and theextracellular fluid (one-third of total body water). See A. Guyton, J.Hall, Textbook of Medical of Physiology. 9^(th) ed., Philadelphia, W.B.Saunders Company (1996)]. The extracellular fluid in turn is dividedinto the interstitial fluid (extravascular) and the blood plasma(intravascular). Water permeable lipid membranes separate thecompartments and water is transferred rapidly between them through theprocess of diffusion, in order to equalize the concentrations of waterand other analytes across the membrane. The net water flux from onecompartment to another constitutes the process of osmosis and the amountof pressure required to prevent osmosis is termed the osmotic pressure.Under static physiological conditions the fluid compartments are atequilibrium. However, during a net fluid gain or loss as a result ofwater intake or loss, all compartments gain or lose water proportionallyand maintain a constant relative volume.

An important mechanism for distributing substances contained in bloodserum that are needed by the tissues, water and glucose, for example, isthrough the process of diffusion. It can be seen that Fick's Law ofdiffusion drives the short-term intra-/extra vascular fluid compartmentbalance. The movement of water and other analytes from intravascular toextravascular compartments occurs rapidly as molecules of water andother constituents, including glucose, in constant thermal motion,diffuse back and forth through the capillary wall. On average, the rateat which water molecules diffuse through the capillary membrane is abouteighty times greater than the rate at which the plasma itself flowslinearly along the capillary. In the Fick's Law expression, the actualdiffusion flux, I_(OA), is proportional to the concentration gradient,dC/dx between the two compartments and the diffusivity of the molecule,D_(A) according to the equation $\begin{matrix}{I_{QA} = {{- D_{A}}\quad{\frac{\mathbb{d}C}{\mathbb{d}x}\quad}_{↤}^{\sqrt{}}}} & (1)\end{matrix}$

Short-term increases (or decreases) in blood glucose concentrations leadto an increase (or decrease) in blood osmolality (number of moleculesper unit mass of water). Fluid is rapidly re-distributed accordingly andresults in a change in the water concentration of each body compartment.In the case of hyperglycemia, the osmotic effect leads to a movement ofextravascular water to the intravascular space compartment where glucoseconcentrations are higher. At the same time, glucose is transported fromthe intravascular space to the extravascular compartment in an effort toequilibrate the osmolality of the two compartments. Conversely, adecrease in blood glucose concentration leads to a movement of water toextravascular space from the intravascular compartment along with themovement of glucose from the extravascular space into the intravascularspace.

Because the cell membrane is relatively impermeable to most solutes buthighly permeable to water, whenever there is a higher concentration of asolute on one side of the cell membrane, water diffuses across themembrane toward the region of higher solute concentration. Large osmoticpressures can develop across the cell membrane with relatively smallchanges in the concentration of solutes in the extracellular fluid. As aresult, relatively small changes in concentration of impermeable solutesin the extracellular fluid, such as glucose, can cause tremendouschanges in cell volume.

Sampling Error

Noninvasive measurement of tissue properties and analytes, such as bloodglucose concentration, may employ NIR spectroscopic methods. S. Malin,T. Ruchti, U.S. Pat. No. 6,280,381, supra, describes a system fornoninvasively predicting blood glucose concentrations in vivo, using NIRspectral analysis. Such NIR spectroscopy-based methods utilizecalibrations that are developed using repeated in vivo optical samplesof the same tissue volume. These successive measurements must yield asubstantially repeatable spectrum in order to produce a usablecalibration. As herein described, the heterogeneous and dynamic natureof living human skin leads to sampling uncertainty in the in vivomeasurement. Sampling differences can arise due to variable chemicalcomposition and light scattering properties in tissue. As an example:because glucose is not uniformly distributed in tissue, a variation inthe volume of tissue sampled is likely to lead to a variation in thestrength of the glucose signal, even though glucose concentration in thetissue or blood remains constant. Variation in the repeated placement ofthe optical probe used for sampling at the measuring surface site canlead to sampling errors in two separate ways: first, variations in thelocation of the probe can cause a different tissue volume to be sampled,and second, varying the amount of pressure applied by the probe on thetissue can alter the optical scattering by the tissue, thereby changingthe sampled tissue volume. A change in optical sampling may lead to avariation in the spectral signal for a target analyte even though theconcentration of the analyte in the blood or tissue remains unchanged.Furthermore, air gaps between the surface of the optical probe and thesurface of the tissue being sampled give rise to variable surfacereflection. Variable surface reflection leads to a variable light launchinto the tissue that in turn gives rise to an increase in nonlinearnature of the spectral measurements. Certainly, a variable nonlinearmeasurement would be very difficult to calibrate.

Various systems for guiding and coupling optical probes are known. Forexample, M. Rondeau, High Precision Fiberoptic Alignment SpringReceptacle and Fiberoptic Probe, U.S. Pat. No. 5,548,674; Aug. 20, 1996and R. Rickenbach and R. Boyer, Fiber Optic Probe, U.S. Pat. No.5,661,843; Aug. 26, 1997 both disclose fiber optic probe guidesutilizing ferrules through which a fiber optic cable or thread islongitudinally threaded. Both devices are connectors that couple fiberoptic cables or threads to receptacles in various forms of medicalequipment, or to other fiber optic cables. Neither device provides ameans for repeatably coupling a fiber optic probe to a tissuemeasurement site.

T. Kordis, J. Jackson, and J. Lasersohn, Systems Using Guide Sheaths forIntroducing, Deploying and Stabilizing Cardiac Mapping and AblationProbes, U.S. Pat. No. 5,636,634; Jun. 10, 1997 describe a system thatemploys catheters and guide sheaths to guide cardiac mapping andablation probes into the chambers of the heart during surgery ordiagnostic procedures. The Kordis teachings are directed to surgicalmethods for the heart, and have nothing to do with optical sampling oftissue in vivo. Furthermore, the apparatus of Kordis, et al. would notbe suitable for repeatably coupling an optical probe to a tissuemeasurement site.

M. Kanne, Laser Mount Positioning Device and Method of Using the Same,U.S. Pat. No. 5,956,150; Sep. 21, 1999 describes a method for using anillumination device, such as a laser to align two components during anassembly process. The Kanne teachings are directed to a manufacturingprocess rather than optical sampling of tissue in vivo. The Kanne devicedoes not provide any means for repeatably placing a probe guide at atissue measurement site. It also has no way of monitoring the surfacetemperature at a tissue measurement site, or of minimizing surfacetemperature fluctuations and accumulation of moisture at a tissuemeasurement site.

D. Kittell, G. Hayes, and P. DeGroot, Apparatus for Coupling an OpticalFiber to a Structure at a Desired Angle, U.S. Pat. No. 5,448,662, Sep.5, 1995 disclose an optical fiber support that is coupled to a frame forpositioning an optical fiber at a desired angular position. As with theprior art previously described, the teachings of Kittell, et al. havenothing to do with optical sampling of tissue in vivo. Furthermore, thedisclosed device allows an operator to immobilize an optical fiber sothat it is maintained in a fixed position, but it does not offer a meansof repeatably coupling a fiber optic probe to a tissue measurement site.It also has no way of monitoring the surface temperature at a tissuemeasurement site, or of minimizing accumulated moisture and temperaturefluctuations at the site.

R. Messerschmidt, Method for Non-Invasive Blood Analyte Measurement withImproved Optical Interface, U.S. Pat. No. 5,655,530, Aug. 12, 1997discloses an index-matching medium to improve the interface between asensor probe and a skin surface during spectrographic analysis.Messerschmidt teaches a medium containing perfluorocarbons andchlorofluorocarbons. Since they are known carcinogens,chlorofluorocarbons (CFC's) are unsuitable for use in preparations to beused on living tissue. Furthermore, use of CFC's poses a well-knownenvironmental risk. Additionally, Messerschmidt's interface medium isformulated with substances that would be likely to leave artifacts inspectroscopic measurements.

There exists, therefore, a need in the art for a means of achieving theprecise optical sampling necessary for developing noninvasivecalibrations for measuring tissue analytes. A solution to the problem ofcontrolling optical sampling during a noninvasive measurement needs toaddress several challenges posed by the structural characteristics anddynamic properties of living tissue, in particular, skin:

-   -   Controlling surface reflection due to optical aberrations in        surface coupling and stretching of the surface tissue;    -   Controlling variations in tissue volume sampled due to imprecise        placement; and variable stretching of dermal collagen, leading        to sampling volume uncertainty;    -   Correcting measurement bias related to water pooling in the        tissue resulting from pressure on the area in the vicinity of        the measurement site from instrumentation or placement guides;        and    -   Stabilizing hydration of surface tissue.

It would be desirable to provide a placement guide for an optical probethat coupled the probe to a tissue measurement site for in vivo opticalsampling of the tissue. It would also be desirable to provide a means ofassuring that the same tissue sample volume may be repeatably sampled,thus eliminating sampling errors due to mechanical tissue distortion andprobe placement. It would also be desirable to provide a way to minimizetemperature fluctuations and stabilize stratum corneum moisture contentat the tissue measurement site, thus eliminating further sources ofsampling error. It would also be highly advantageous to provide anoptical coupling medium to provide a constant interface between anoptical probe and the skin at a tissue measurement site that isnon-toxic and non-irritating and that doesn't introduce error intospectroscopic measurements. Additionally, it would be advantageous toprovide a means of monitoring surface temperature at the tissuemeasurement site, therefore assuring that the temperature remainsconstant across repeated optical samples. Finally, it would beadvantageous to provide a means for correcting the tissue sampling biasthat results from the uncertainty inherent to the mechanical attachmentprocess used to install the placement guide at the measurement site.

Summary of the Invention

The invention provides an optical sampling interface system thatminimizes and compensates error resulting from sampling variation and/orstate fluctuations at a measurement site during optical tissue samplingand subsequent analyte measurement by spectroscopic means.

An optical probe placement guide facilitates repeatable locationaccuracy on the surface of a tissue measurement site with a minimal andrepeatable degree of tissue distortion and displacement. The majorstructural component of the probe placement guide is a mount having anaperture, into which the optical probe is received during use. Inaddition to improving the precision of probe placement during the courseof multiple measurements, the guide aperture induces the formation of atissue meniscus, created by the pooling of epidermal water in the guideaperture due to the relative difference in the contact pressure at theguide adhesion surface and the guide aperture, where no part of theguide contacts the tissue. The formation of the tissue meniscusminimizes interference due to surface irregularities and controlsvariation in the volume of tissue sampled.

An occlusive element placed over the tissue meniscus isolates the tissuemeniscus from environmental fluctuations, thus stabilizing the degree ofhydration of the tissue meniscus and thereby stabilizing surface tensionof the tissue meniscus. An optical coupling medium placed on the surfacetissue at the tissue measurement site eliminates sampling errors due toair gaps between the skin surface and the optical probe.

A measurement and bias correction element applies a bias correction tospectral measurements, and the associated analyte measurement. Such biascorrections are performed identically for all data taken over the courseof one guide placement. When the guide is removed and replaced, a newbias correction is determined for all subsequent data taken with thesecond guide placement.

Additionally, each of the separate elements of the invented system canbe individually deployed, as standalone solutions to counter varioussources of measurement error. Thus, the probe placement guide,independent of the other elements of the system, provides a significantreduction in sampling error; the occlusive element provides asignificant reduction in measurement error due to state fluctuations atthe surface of the measurement site; and the correction algorithm can beapplied to spectral measurements in settings lacking the other elementsof the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows second derivative absorbance spectra of excised human skin,pure fat and pure collagen;

FIG. 2 shows an optical probe placement guide according to theinvention;

FIG. 3 provides a block diagram of a measurement and bias correctionsystem according to the invention;

FIG. 4 shows an optical probe and a tissue measurement site opticallycoupled by a layer of an optical coupling fluid according to theinvention.

FIGS. 5-7 show plots of measurement variation attributable to samplingerror without and with the invention.

DETAILED DESCRIPTION

In spectroscopic analysis of living tissue, it is often necessary tooptically sample the same tissue volume repeatedly though the use of anoptical probe; for example while developing a noninvasive calibrationfor measuring one or more tissue analytes, and subsequently, when takingmeasurements for the actual analyte measurement. Sampling errors can beintroduced into these measurements because of the difficulty ofrepeatedly placing the optical probe at the precise location used inpreceding measurements, and repeatably producing the same nominal degreeof tissue distortion and displacement. With each small variation in thelocation of the probe, or variations in the amount of pressure resultingfrom the repeated probe contact events, a slightly different tissuevolume is sampled, thereby introducing sampling errors into themeasurements. The invention provides an optical sampling interfacesystem that eliminates or minimizes factors that account for samplingerror.

Probe Placement Guide

A system is described herein that provides superior sampling precisionof the target tissue volume through the use an optical probe placementguide that is removably attached to the tissue site to achieve the goalof highly repeatable probe placement at a targeted tissue measurementsite. A key characteristic of the guide is that it provides a means forregistering the location of the targeted tissue volume with respect tothe optical probe such that a particular tissue volume is preciselysampled by the optical system. Registration refers to providing feedbackregarding the position of the optical probe relative to a targetlocation on the tissue. The means for registering between the guide andthe optical probe may be mechanical, optical, electrical or magnetic. Inaddition, the guide includes an aperture into which the optical probe isreceived. The aperture serves several purposes including those of:

-   -   a mechanical registration point;    -   a means for creating a stable tissue meniscus; and    -   an opening for receiving an occlusion plug so that the surface        state of the tissue at the measurement site may be stabilized        between measurements.

In the preferred embodiment, shown in FIG. 2, the guide 200 is oval andcontoured to approximate the surface of the sampled tissue site, forexample, the volar (that corresponding to the palm of the hand) ordorsal surface of the forearm. However, other shapes are used for otherlocations of the body such as the hand, the earlobe, the leg, theabdomen, the upper arm region and the fingers. The design of the guideis intended to allow for comfortable and unobtrusive use withoutapplication of significant mechanical energy to the sampled tissue site.In the current embodiment, the guide is composed of a rigid polymer,allowing for the creation of a stable tissue meniscus. However, othermaterials providing the requisite combination of rigidity and lightweight, such as lightweight metals, would also be suitable.

Attachment of the guide to the tissue site may be by means of anadhesive layer 201 on the contact surface of the guide 200. The adhesivelayer may be applied at the time of manufacture, or it may be applied tothe guide prior to usage. Generally, the adhesive covers the entirecontact surface of the guide, that surface of the guide that is incontact with the skin area adjacent to and surrounding the tissuemeasurement site. Additionally, other attachment means are suitable suchas straps, suction, or armbands. The guide is attached to the tissuesite at the beginning of a measurement period. Typically this period isthe beginning of a particular day after a previously used guide has beenremoved. In the preferred embodiment, the method of attachment is toplace the guide 200 onto a noninvasive measurement device with theadhesive layer in place and exposed. The tissue measurement site is thenplaced onto the guide with a rough registration through an arm cradle orelbow and wrist supports. During this first placement, the guide becomesaffixed to the tissue site.

The guide 200 allows for the distribution of mechanical energytransferred from the instrument to the arm over a greater area aroundthe measurement site. However, in applications involving a portion ofthe body subject to deformation or movement, the guide may be composedof a flexible material, such as a flexible polymer, that provides for astabilization of the measurement site and deformation of the underlyingtissue without applying undue force to the targeted tissue volume.

The guide has an aperture 202, into which an optical probe is received.The sizes and shapes of the optical probe and the guide aperture 202 arematched to each other such that when the optical probe is received bythe guide, it fits snugly and provides a mechanical registration in thex-y plane relative to the tissue measurement site. To avoidover-penetration of the optical probe into the tissue and to promote arepeatable pressure between the optical probe and the tissue, the guideand the optical probe are equipped with mechanical stops 203 that limitand control the penetration of the optical probe into the tissue (thez-direction). The weight of the tissue is transferred to the opticalprobe through the mechanical stop 203 and thereby reduces the pressureat the tissue measurement site.

The guide is equipped with a slot 208 for the optional insertion of atemperature probe. This feature is particularly useful during thecalibration phase for monitoring of skin temperature.

Measurement Site Occlusion

When the tissue site is not being interfaced to the optical probe anocclusion plug 204 is normally inserted into the aperture 202. Theocclusion plug penetrates into the aperture to the same extent as theoptical probe and thereby creates a stable tissue state by simulatingthe contact energy of the optical probe. As discussed previously, theocclusion plug is composed of a material that provides a hydrationbarrier, thus promoting the full and stable hydration of the stratumcorneum. In the preferred embodiment, the plug is composed of the samematerial as the guide and possess a mechanical stop 205 to control thepenetration into the tissue site. The size of the portion of the plugthat is inserted into the aperture 206 is matched to the portion of theoptical probe that is received by the guide aperture 202. Attachment ofthe plug to the guide may be through the use of one or more magnetslocated in both the guide and plug assemblies 207. However, othermethods of attachment may be used, such as VELCRO, adhesives and snaps.Alternately, the plug can be composed of a material that is elastic innature and is kept in place by virtue of its tight fit into the guideaperture. Also, the plug can be a hydrophobic material, such ascellophane.

From the foregoing, one of ordinary skill in the art will recognize thatan important aspect of the optical sampling system is the maintenance ofan optimal level of hydration of the surface tissue at the measurementsite for enhancement of the optical signal, sample reproducibility, andsuppression of surface reflectance. As previously described, thepreferred embodiment of the hydration mechanism is by occlusive blockageof trans-epidermal water loss (TEWL). This blockage ensures a steadystate hydration as water diffusing from interior tissue is trapped inthe stratum corneum. Attainment of high hydration levels reduces thewater concentration gradient that provides the driving force for thistrans-epidermal water movement. Thus, the above described occlusive plugfits snugly into the guide aperture during periods between measurements,acting to insulate the tissue in the guide aperture from trans-epidermalwater loss and the environmental effects of temperature and humiditythat are known to influence the stratum corneum hydration state. Inaddition to the preferred embodiment just described, in alternateembodiment, wrapping a flexible polymer sheet (an occlusion patch)around the measurement site may also be used to attain a highly hydratedstate via occlusion.

Other solutions to the problem of maintaining hydration of the stratumcorneum, consistent with the spirit and scope of the invention arepossible, including, but no limited to:

-   -   a vapor barrier or semi-permeable membrane (for example,        GORE-TEX, manufactured by W. L. Gore and Associates of Newark        Del. as the mount) in the form of a wrap or a patch configured        to cover the site target for measurement. In this latter        embodiment the “patch” is affixed to the tissue site through an        adhesive or other attachment mechanism such as a strap or a        wrap;    -   non-occlusive mechanisms for hydration of the stratum corneum        may also be used, including:        -   an application of water that is pneumatically driven into            the skin;        -   ultrasound energy applications to accelerate passive            occlusion; and        -   topical application of skin toners and other water/solute            mixtures such as alpha hydroxy acid solutions that serve to            drive water and solute into the dry outer skin layer.        -   topical analgesic formulations that enhance and/or stimulate            local circulation at the measurement site leading to an            improvement in surface hydration.

The mechanisms for achieving stratum corneum hydration may also be usedin coupled treatments. For example: Skin toner solution or an ultrasoundenergy application may be used in conjunction with an occlusive plug.

After an initial measurement is made, as described above, subsequentmeasurements are made by simply placing the tissue site onto thenoninvasive measurement device (after removing the occlusion plug) andallowing the guide to provide mechanical registration. After the opticaltissue measurement is performed, the tissue is taken away from thedevice and the occlusion plug is re-inserted.

Optical Registration

In an alternate embodiment, the guide provides a means for opticalregistration. In this embodiment, reflectors or light sensitive elementsare placed onto the guide. The optical probe assembly is equipped withlight sources and several detectors that allow the position of the guideto be accurately assessed, in either two or three dimensions. In a firstconfiguration, two dimensions (x,y) are assessed and a mechanical stopis used to control the third dimension. In a second configuration, thelocation of the guide is optically assessed in all three dimensions(x,y,z). Because the position of the guide is constant with respect tothe targeted tissue volume, the positional assessment provides accurateinformation regarding the location of the targeted tissue volume withrespect to the optical probe. The registration information provided bysuch assessment is used to place the tissue site onto the optical probe,or vice versa, through any of the following means:

-   -   an operator or user is given a visual or audible signal        indicating how to move the tissue site with respect to the        optical probe;    -   a mechanical positioning system is used to position the tissue        measurement site with respect to the optical probe; or    -   a mechanical positioning system is used to position the optical        probe onto the tissue measurement site.

One skilled in the art will appreciate that a magnetic sensing systemcan also be readily applied for assessment of the location of the guidewith respect to the tissue measurement site.

In addition to improving the precision of the probe placement eventduring the course of multiple measurements, the guide aperture inducesthe formation of a tissue meniscus, an upward bulge of tissue into theoptical probe aperture. The tissue meniscus, a pooling of subsurfacewater in the guide aperture resulting from a relative difference in thecontact pressure at the guide adhesion surface and the guide aperture,both provides for limitation of the penetration of the probe into thetissue and guarantees a highly compliant and energy absorbing contactevent.

The hydrostatic pressure within the tissue in the aperture is greaterthan that on the nude (guideless) tissue sample. This increasedhydrostatic pressure absorbs energy translated to the tissue when theprobe contacts the tissue, thus limiting the resulting distortion ofdermal collagen tissue. Distortion of dermal collagen has a strongeffect on the tissue optical properties and thus the sampled tissuevolume. In order to achieve this correction, the termination of theoptical probe should be flush with the contact surface at the tissuemeasurement site when the optical probe is fully seated.

Optical Coupling Medium

The interface between the optical probe and the skin surface at thetissue measurement site can also be a significant source of samplingerror. Since the underlying tissue is not homogenous, the surface skinat the tissue measurement site may be uneven, with frequentirregularities. Coupling the relatively smooth surface of the opticalprobe with the irregular skin surface leads to air gaps between the twosurfaces. The air gaps create an interface between the two surfaces thatadversely affects the measurement during optical sampling of tissue. Asshown in FIG. 3, an amount of an optical coupling medium such as anoptical coupling fluid 401 between the optical probe 402 and the skin ofthe tissue measurement site 400 eliminates such gaps.

Preferably, the optical coupling fluid:

-   -   is spectrally inactive;    -   is non irritating and nontoxic; and    -   has low viscosity for good surface coverage properties.    -   and has poor solvent properties with respect to leaching fatty        acids and oils from the skin upon repeated application.

It is possible to achieve such characteristics by selecting the activecomponents of the optical coupling fluid from the class of compoundscalled perfluorocarbons, those containing only carbon and fluorineatoms. Nominally limiting chain length to less than 20 carbons providesfor a molecule having the requisite viscosity characteristics. Themolecular species contained in the perfluorocarbon coupling fluid maycontain branched or straight chain structures. A mixture of smallperfluorocarbon molecules contained in the coupling fluid aspolydisperse perfluorocarbons provides the required characteristicswhile keeping manufacturing costs low.

In a preferred embodiment, the optical coupling fluid is a perfluorocompound such as those known as FC-40 and FC-70, manufactured by 3MCorporation. Such compounds are inactive in the Near IR region,rendering them particularly well suited for optical sampling proceduresemploying Near IR spectra. Additionally, they have the advantage ofbeing non-toxic and non-irritating, thus they can come into directcontact with living tissue, even for extended periods of time, withoutposing a significant health risk to living subjects. Furthermore,perfluoro compounds of this type are hydrophobic and are poor solvents;therefore they are unlikely to absorb water or other contaminants thatwill adversely affect the result during optical sampling. It ispreferable that the optical sampling fluid be formulated without theaddition of other substances such as alcohols or detergents, which mayintroduce artifacts into the optical sample. Finally, the exceptionalstability of perfluoro compounds eliminates the environmental hazardcommonly associated with chlorofluorocarbons.

Other fluid compositions containing perfluorocarbons andchlorofluorocarbons are also suitable as optical coupling fluids: forexample a blend of 90% polymeric chlorotrifluroethylene and 10% otherfluorocarbons would have the desired optical characteristics.Chlorotrifluorethene could also be used. While these compositions havethe desired optical characteristics, their toxicity profiles and theirsolvent characteristics render them less desirable than the previouslydescribed perfluoro compounds.

Additionally, other fluid media are suitable for coupling of an opticalprobe to a tissue measurement site, for example, skin toner solutions oralpha hydroxy-acid solutions.

During use, a quantity of optical sampling fluid is placed at theinterface of the tissue measurement site and the fiber optic probe sothat the tissue measurement site and the fiber optic probe may betightly optically coupled without leaving any air spaces between the twosurfaces. In practice, one convenient way of placing the quantity of theoptical sampling fluid at the interface between the tissue measurementsite and the probe is to place a small amount of the fluid on the skinsurface prior to placing the fiber optic probe, although it is easier toplace it on the fiber-optic probe.

Furthermore, certain non-fluid media having the requisite opticalcharacteristic of being near-IR neutral are also suitable as opticalcoupling media, for example, a GORE-TEX membrane interposed between theprobe and the surface of the measurement site, particularly when used inconjunction with one of the fluid media previously described.

Bias Corrections

Finally, a bias correction is preferably made to the measurement toaccount for variations in the size of the meniscus caused by the guideinstallation. These bias corrections are applied to the processedspectral measurement and to the predicted analyte value just prior toprediction

An embodiment of a bias correction system 300 associated with the guideapparatus is summarized in FIG. 3. A non-invasive measurement system 301provides a “tissue measurement” (302), mε

^(1×N) where N corresponds to the dimensionality of the measurement. Inthe preferred embodiment, m refers to the intensity spectrum of thetissue sample represented by the intensity at N wavelengths (orwavelength ranges or selected wavelengths) selected from a wavelengthrange, for example 700-2500 nm. In the preferred embodiment, abackground or reference, m_(o), is used to standardize or normalize thetissue measurement according to the calculation $\begin{matrix}{{a = {{- \log_{10}}{\frac{m}{m_{o}}\quad}_{↤}^{\sqrt{}}}},} & (2)\end{matrix}$where m_(o) is an estimate of light incident on the sample, m is anintensity spectrum of light detected and a is analogous to an absorbancespectrum containing quantitative information that is based on the knowninteraction of the incident light with components of the body tissue.Alternately, the tissue measurement, m, can be used directly instead ofa.

The standardized tissue measurement, a, is preferably preprocessed 303to attenuate noise and to reduce the interference related to surfacereflectance, tissue volume distortion and instrumental effects toproduce the processed tissue measurement, x. In the preferred embodimentthe preprocessing steps include calculating the first derivative,selecting specific wavelengths and wavelength regions specific to theanalyte of interest and scatter correction (e.g., multiplicative scattercorrection).

A bias correction step 304 follows the preprocessing steps defined abovethrough the determination of the difference between the preprocessedestimated tissue background—the tissue template 305, and x throughz=x−(cx _(t) +d)  (3)where x is the preprocessed tissue measurement or the selected set offeatures, x_(t) is the estimated background or tissue templateassociated with the current guide placement, and c and d are slope andintercept adjustments to the tissue template. After each guideplacement, the tissue template 305 is determined through one or moretissue measurements (after preprocessing) and a data selection criterion(for example, by selecting only tissue measurements that resemble eachother closely and averaging them). In the preferred embodiment, x_(t) iscalculated from a single tissue measurement that is collected after anequalization period following the placement of the guide and c=1 andd=0. This process is referred to as “bias correction” and involves both:

-   -   the collection of one or more tissue measurements that are        processed to form a tissue template; as well as    -   an associated set of reference analyte values determined from a        primary analyte measurement source.

For example, in the case of near-infrared measurement of glucose, thereference analyte values are determined from an electrochemical analysisof blood draws. The analyte values are combined, according to the samestrategy as that used to create the tissue template to form an analytemeasurement bias adjustment 309, b, through the equationŷ=g(z)+b  (4)where g:

^(M)→

¹ is a calibration model 307 used to map z to an estimate of the targetanalyte 308. The model is determined from a calibration set of exemplarypaired data points each consisting of a pre-processed and bias correctedtissue measurement (z) and an associated reference analyte value (y)determined from an analysis of a blood or interstitial fluid sample.According to this process, blood, serum, plasma, or interstitial drawsare taken from a tissue site that is either near the sensor sample siteor has been designed/determined to reflect the sample site. For example,when non-invasive near-infrared measurements for the purpose of glucosemeasurement are taken for calibration on the forearm, it is possible insome individuals to collect a capillary blood draw from the same forearmor an alternate site such as opposite forearm. Alternately, rather thanusing blood draws, it is beneficial in some instances to useinterstitial glucose values rather than capillary glucose values. Themethod for designing the structure of g is through the process of systemidentification [L. Ljung, Systems Identification: Theory for the User,2d.ed., Prentice Hall (1999)]. The model parameters are calculated usingknown methods including multivariate regression or weighted multivariateregression [N. Draper, H. Smith, Applied Regression Analysis, 2d.ed.,John Wiley and Sons, New York (1981)], principal component regression[H. Martens, T. Naes, Multivariate Calibration, John Wiley and Sons, NewYork (1989)], partial least squares regression [P. Geladi, B. Kowalski,Partial least-squares regression: a tutorial, Analytica Chimica Acta,185, pp.1-17, (1986)], or artificial neural networks [S. Haykin, NeuralNetworks: A Comprehensive Foundation, Prentice Hall, Upper Saddle RiverN.J. (1994)]. Calibration data must also be bias corrected if datacontains subsets associated with different guide placement events.

Optionally, the bias corrected tissue measurements undergo an outlierdetection step 306. As indicated in FIG. 3, the necessity for outlierdetection, and the form of an outlier detection procedure are dependenton the sampling technology employed. Outlier detection provides a methodof detecting invalid measurements through spectral variations thatresult from problems in the instrument, poor sampling of the subject ora subject outside the calibration set. One method of detecting outliersis through a principal component analysis and an analysis of theresiduals.

EXEMPLARY APPLICATIONS Example 1

A study was performed to examine the difference in spectral variationbetween several different near-infrared sampling treatments on a singlesubject. Near-infrared spectra were collected using a custom builtscanning near-infrared spectrometer that collected intensity spectra indiffuse reflectance over the wavelength range 1100-1950 nm. The spectralsampling interval was one nanometer and the signal-to-noise ratio at thepeak intensity was approximately 90 dB. The detector used in the studywas Indium-Gallium-Arsenide (InGaAs) and the optical configurationconsisted of a simple fiber optic interface to the skin with a small (<2mm) distance between the illumination and detection fibers. Referencespectra were recorded before each sample measurement by scanning a 99%SPECTRALON reflectance material provided by LABSHPERE of North SuttonNH. The absorbance spectrum was calculated through Equation (2), supra.

Approximately twenty near-infrared absorbance spectra were collected onthe subject's forearm using the following treatments:

-   -   1. Baseline measurements using only elbow and wrist supports to        guide the patient's arm placement;    -   2. Measurements were taken using the preferred embodiment of the        guide positioning system herein described, without occlusion of        the measurement site; and    -   3. Both the guide positioning system and the disclosed method of        occlusion (a plug in the aperture of the guide).

Before the collection of each spectrum, the subject's arm was replacedon the optical probe. Analysis of the data was performed on each of thethree data subsets described above and consisted of calculating the rootmean square variation at each motor position of the spectrometer. A plotof the normalized RMS variation versus motor position is given in FIG.5. As shown, the plot 501 of RMS variation without the guide positioningsystem shows relatively more sample variation. As the plots 502, 503,respectively, indicate, the relative variation related to replacement ofthe subjects arm on the optical probe is reduced by utilization of theguide (Control 1) and still further reduced through the addition of siteocclusion (Control 2).

Example 2

As a further illustration of the benefit of the guide placement system,sixty measurements were performed on a single subject with and withoutthe guide positioning system. All spectra were collected using a custombuilt scanning near-infrared spectrometer. The instrument collectedintensity spectra in diffuse reflectance from the forearm in thewavelength range 1050-2450 nm. The spectral sampling interval was 1 nmand the signal-to-noise ratio at the peak intensity was approximately 90dB. The detectors used in the study were a combination ofIndium-Gallium-Arsenide (InGaAs) and extended InGaAs detectors. Theoptical configuration consisted of a simple fiber-optic interface to theskin with a small (<2 mm) distance between the illumination anddetection fibers. Reference spectra were recorded prior to each samplemeasurement by scanning a 99% SPECTRALON reflectance material andabsorbance was calculated according to Equation (2). A cradle wasdeveloped to position the arm over the sample interface in areproducible location with a reproducible degree of pressure, with thesubject remaining seated during the experiment. In the first set ofmeasurements, 60 samples were collected, each representing a differentarm placement; and absorbance was calculated. In the second set ofmeasurements, 60 samples were collected with the use of the guidepositioning system. The absorbance spectra, shown plotted in FIG. 2,illustrate the benefit of using the guide positioning system. The plotof FIG. 6A shows the absorbance spectra over the 60 arm placementswithout the use of the guide positioning system. When the guide wasused, the amount of spectral variation is significantly reduced (FIG.6B).

Example 3

As a test of the benefit of the method of occlusion, 60 measurementswere performed on a single subject using the apparatus described inExample 2. In the first set of measurements, 60 samples were collectedusing the guide positioning system without occlusion and absorbance wascalculated as previously described. In the second set of measurements,60 samples were collected with the use of both the guide positioningsystem and the preferred method of occlusion (a plug in the guideaperture). FIG. 6 shows the absorbance spectra collected withoutocclusion (FIG. 6A) and the absorbance spectra collected after occlusion(FIG. 6B). The decrease in surface variation associated with the waterbands demonstrated the improved optical sampling realized as a result ofthe method of occlusion.

While the invented optical probe placement guide allows highlyrepeatable probe placement at a targeted tissue measurement site, theinvention may also be used to produce small sampling variations in acontrolled manner by shifting the placement of the optical probe inknown increments across successive optical samples.

The invention provides a means of limiting sampling errors during invivo spectroscopic examination of tissue samples by providing highlyrepeatable optical probe placement at a targeted tissue measurementsite. Structural features of the invention minimize temperaturefluctuations and variable stratum corneum hydration at the tissuemeasurement site and on the optical probe, and variations in tissuedistortion and displacement, all sources of sampling error. An optionaltemperature probe in direct contact with the skin surface at the tissuemeasurement site allows the monitoring of skin temperature acrosssuccessive measurements. An optical coupling fluid eliminates air spacesat the interface of the skin surface of the tissue measurement site andthe optical probe. A fully hydrated stratum corneum is attained by theuse of an occlusive plug or other mechanism. Finally, spectralmeasurements, and resulting analyte measurements are bias corrected tocompensate error resulting from guide placement.

While the invented optical sampling interface system has been hereindescribed in relation to optical sampling of tissue, one skilled in theart will appreciate that the invention may be applied in other settingsrequiring repeatable placement of an optical probe.

It is understood that each of the elements of the optical probeplacement guide measurement system herein described are individuallybeneficial to the measurement and therefore can be used with or withoutthe other elements. Specifically, the guide, the hydration controlsystem, the coupling fluid, and the bias correction are uniquelybeneficial. For example, in the event that an alternate mechanicalpositioning system is developed, the hydration control process, biascorrection, and the coupling fluid are still beneficial.

Although the invention is described herein with reference to certainpreferred embodiments, one skilled in the art will readily appreciatethat other applications may be substituted for those set forth hereinwithout departing from the spirit and scope of the present invention.Accordingly, the invention should only be limited by the Claims includedbelow.

1. A method of compensating measurement bias in noninvasive analytemeasurement, said bias resulting from variations in sampled volume andmeasurement conditions between measurements, comprising the steps of:providing a tissue measurement; determining difference between saidtissue measurement and a tissue template; mapping resulting differenceto a measurement of a target analyte according to a calibration model;and applying a baseline adjustment to said analyte measurement.
 2. Themethod of claim 1, wherein said variations in sampled volume result fromdifferences in placement of either an optical probe, or an optical probeplacement guide between measurements.
 3. The method of claim 1, furthercomprising the step of preprocessing said tissue measurement and tissuetemplate before determining the difference between said tissuemeasurement and tissue template, preprocessing comprising any of:derivative calculation; selecting wavelengths and wavelength regionsspecific to the target analyte; and scatter correction.
 4. The method ofclaim of claim 1, wherein said step of determining difference betweensaid tissue measurement and a tissue template comprises: determiningdifference between the tissue template and a preprocessed spectrumaccording to:z=x−(cx _(t) +d); wherein x comprises a pre-processed spectrum or aselected set of features, x_(t) comprises a tissue template associatedwith a measurement period.
 5. The method of claim 4, wherein said tissuetemplate is determined through one or more tissue measurements combinedaccording to a predetermined data selection criterion during eachmeasurement period.
 6. The method of claim 4, wherein c=1 and d=0. 7.The method of claim 1, further comprising the step of: providing anassociated set of reference analyte values, said values combinedaccording to said predetermined data selection criterion to form ameasurement bias adjustment.
 8. The method of claim 7, wherein saidreference analyte values are determined from an analysis of a blood orinterstitial fluid sample.
 9. The method of claim 7, wherein saidanalyte values are combined according to same strategy used to createsaid tissue template, wherein an analyte bias adjustment, b, is formedaccording toŷ=g(z)+b; where g:

^(M)→

¹ is a calibration model used to map z to an estimate ŷ, of the targetanalyte.
 10. The method of claim 9, wherein said model is determinedfrom a calibration set of exemplary paired data points each consistingof a pre-processed and bias corrected tissue measurement (z) and anassociated reference analyte value (y) determined from an analysis of ablood or interstitial fluid sample.
 12. The method of claim 1, whereinsaid measurement site is on any of: a dorsal surface of a forearm; avolar surface of a forearm; and a torso.
 13. The method of claim 1,further comprising the optional step of detecting outliers.